A common process step in the manufacture of integrated circuits is the deposition of one or more metallic layers onto the surface of a semiconductor wafer or substrate. Typically, holes, known as trenches and vias, exist on the surface of, and extend into, the substrate. The interior walls of these holes create vertical, or nearly vertical, surfaces with respect to the horizontal surfaces of the top of the substrate and the bottom of each hole. Both these vertical surfaces and the horizontal surface must be covered with a thin film of metal. Several processes have evolved to accomplish the metal film deposition step.
Although several processes exist to form the metal films, a typical process involves physical vapor deposition--or "sputter deposition." In such a process, one or more metal layers are deposited onto a substrate. A typical sputtering process begins with a "target" comprising the material which the operator wishes to deposit onto a substrate. This target is then bombarded with ions to form a vapor comprised of the freed atoms of the target. The vapor is then allowed to contact the substrate and form the desired thin film. Using conventional sputter deposition processes, the vapor typically reaches temperatures of 250.degree. C. to 430.degree. C.
One example of the current processes used for sputter deposition is disclosed in U.S. Pat. No. 5,597,458, issued to Sanchez, Jr. et al. That patent discloses a sputtering process in which an aluminum-copper (Al--Cu) alloy is deposited onto a substrate. Because there are two main phases of copper-aluminum in the alloy, namely CuAl.sub.2 and an aluminum rich "matrix" phase which forms the actual (and desired) thin film, it is an object of the '458 patent to reduce the formation of the CuAl.sub.2 phase. Specifically, the '458 patent seeks to provide a process for controlling the size, distribution, and morphology of the CuAl.sub.2 phase during the sputter deposition of Al--Cu films. According to the '458 patent, to prevent the solid state diffusion of copper, and thereby reduce the formation of CuAl.sub.2, the temperature of the wafer is maintained at about room temperature, specifically about 25.degree. C. By maintaining this temperature during deposition, the copper in the deposited film is less likely to diffuse and form the undesired, more thermodynamically stable, CuAl.sub.2.
Operating the deposition process at room temperature is generally effective in cases like the one above. Several problems have arisen, however, with regard to thin film continuity. The temperature of the substrate during copper thin film deposition significantly affects the ability of the copper to form a continuous film.
Under current typical manufacturing conditions, when the substrate is allowed to reach temperatures of 100.degree. C. or higher, the copper particles agglomerate. Agglomeration is the build-up of metal particles when they strike other metal particles and bind to each other. Consequently, poor plating and uneven seedlayer continuity results. When the seedlayer is discontinuous, the subsequent plating steps result in discontinuous and other non-uniform, undesired, surface irregularities. This is particularly noticeable where trenches and voids are present in the substrate, and especially where those trenches and voids have high aspect ratios. The aspect ratio is the ratio of the longer side to the shorter side of the opening. In the case of "deep" voids and trenches, the aspect ratio is the ratio of the depth of the opening to the width of the opening. Thus, a trench or void with a high aspect ratio is one which is relatively deep and not relatively wide. A square trench has an aspect ratio of 1.
High aspect ratio structures make it particularly difficult to produce an even and continuous copper thin film. This is so because, during conventional deposition processes, copper atoms tend to agglomerate at the trench opening and along the trench walls and effectively "clog" the via or trench. The result is a trench which is unevenly coated, or not coated, in certain areas. That problem is then compounded when a secondary layer is applied, such as an electrolytic plating layer.
The deficiencies of the conventional manufacturing techniques show that a need still exists for a process and apparatus which will effectively and reliably sputter a continuous, relatively defect-free layer of copper onto a substrate, especially a substrate with high aspect ratio structures. Therefore, one object of the present invention is to provide a process and apparatus to effectively sputter a copper seedlayer onto a substrate while preventing the copper from agglomerating. Another object of the present invention is to provide an effective process and apparatus to effectively sputter a copper seedlayer onto a substrate having high aspect ratio structures while preventing the copper from agglomerating.